1. Field of the Invention
The present invention relates to a liquid crystal panel and a liquid crystal projector using the liquid crystal panel.
2. Description of the Related Art
In recent years, a technique for fabricating a semiconductor device in which a semiconductor thin film is formed on an inexpensive glass substrate, such as a thin film transistor (TFT), has been rapidly developed. The reason is that demand for an active matrix type liquid crystal panel has risen.
The active matrix type liquid crystal panel (liquid crystal panel) is such that a thin film transistor (pixel TFT) is disposed for each of several tens to several million pixel portions disposed in matrix form and an electric charge going in and out of each pixel electrode is controlled by a switching function of the pixel TFT.
Besides, particularly, a projection type display device using a liquid crystal panel, a so-called projector has rapidly increased its market. This is because, as compared with a projector using a CRT, the liquid crystal projector has excellent color reproducibility, is small and lightweight, and has low power consumption.
The liquid crystal projector is classified into a three-plate type and a single plate type according to the number of liquid crystal panels to be used.
As compared with the three-plate type liquid crystal projector, since optical parts of the single plate type liquid crystal projector is ⅓ thereof, it is superior in the cost, size and the like. However, in the case where the same liquid crystal panel is used in the three-plate type and a conventional single plate type, while three colors are overlapped with each other on one pixel in the three-plate type, the single plate type can merely use one pixel as a pixel of one color, so that the single plate type is inferior to the three-plate type in picture quality. Further, in the single plate type liquid crystal projector, an unnecessary component of white light from a light source is absorbed by a color filter so that an image of a desired color is obtained. Thus, only ⅓ of white light incident into the liquid crystal panel is transmitted, and usage efficiency of light is low.
For the purpose of improving the brightness of the single plate type liquid crystal projector, although a method of making the light source bright has been adopted, there have occurred problems of heat generation due to light absorption of a color filter and light resistance.
Then, for the purpose of overcoming the defects of the conventional single plate type liquid crystal projector, a three-plate type liquid crystal projector using three dichroic mirrors was devised.
Reference will be made to FIG. 14. FIG. 14 is a structural view of an optical system of the three-plate type liquid crystal projector. Reference numeral 1401 designates a light source made of a lamp and a reflector. White light having a spectrum of red, green and blue is emitted from the light source 1401. The light source 1401 is set so that the parallelism of the emitted white light becomes high. The reflector is used for effectively use the white light emitted from the lamp.
The white light emitted from the light source 1401 enters dichroic mirrors 1402 and 1403. These two dichroic mirrors 1402 and 1403 split the white light from the light source 1401 into lights of three primary colors (red, green, blue).
The dichroic mirror 1402 reflects only the light of a blue (B) wavelength region and allows other lights to be transmitted. The dichroic mirror 1403 reflects only the light of a red (R) wavelength region in the lights transmitted through the dichroic mirror 1402, and allows other lights to be transmitted. A total reflection mirror 1404 reflects the light of a green wavelength region transmitted through the dichroic mirrors 1402 and 1403. By adopting such structure, the white light emitted from the light source 1401 can be split into the three primary colors.
The blue and green lights split by the dichroic mirror 1402 and the total reflection mirror 1404 are reflected by total reflection mirrors 1406 and 1405, and respectively enter liquid crystal panels 1407 and 1409. The red light split by the dichroic mirror 1403 enters a liquid crystal panel 1408. The transmitted lights of blue, red and green transmitted through the liquid crystal panels 1407, 1408 and 1409 are condensed by a dichroic prism 1410 and are projected on a screen by a projection lens 1411.
In recent years, the liquid crystal projector is required to become thin and lightweight, and at the same time, to achieve high fineness, high picture quality, and high brightness.
For the purpose of making the liquid crystal projector thin and lightweight, it becomes necessary to miniaturize a substrate size of the liquid crystal panel. In order to decrease the substrate size and not to degrade the picture quality, the area of a pixel portion must be inevitably reduced by reducing a pixel pitch.
FIG. 15 is a schematic view of a pixel of a liquid crystal panel. A wiring 12, a pixel TFT 15 including an active layer 13 and a gate electrode 14 as a part of the wiring 12 and a pixel electrode 16 are provided as shown in FIG. 15. A black matrix 17 covering a region which is not required to transmit visible light is provided over the wiring 12 and the pixel TFT 15. The black matrix (BM) indicates a film having a light shielding property and provided over a wiring, a pixel TFT, and the like which are not required to transmit visible light.
A pixel pitch L indicates a shorter one of distances between the wirings 12 opposite to each other through a pixel 11. In the case where the distances between the wirings 12 opposite to each other are such that the distance between the wirings in the row direction is equal to that in the column direction, the pixel pitch indicates a distance between the wirings in both.
It is difficult to reduce the thin film transistor (pixel TFT) for driving a liquid crystal and the wiring at the same scale as a reduction in the pixel pitch.
If the thin film transistor is made excessively small, the amount of flowing current is limited. Thus, if the pixel TFT is excessively small, it becomes difficult to cause a current necessary for driving the liquid crystal to flow. Besides, if the wiring is made excessively, thin, the resistance of the wiring becomes large. For this reason, there is a limit in the reduction of the pixel TFT and the wiring.
Thus, if the pixel pitch is made small, the ratio of a portion covered with the BM, such as the pixel TFT and the wiring, to the pixel becomes large, and an opening ratio is lowered.
When the opening ratio is lowered, the brightness of an image is lowered unless the brightness of a light source is raised. However, if the brightness of the light source is raised, consumed electric power becomes large, which is not preferable.
Then, in order to raise the brightness of the image without raising the brightness of the light source, it is conceivable that a microlens array is formed at the side of the liquid crystal panel where light enters.
The microlens array shown here includes a plurality of microlenses on one-on-one basis with respect to the respective pixels. By the microlens array, the light originally blocked off by the black matrix is condensed to the part of the pixel portion through which the visible light is transmitted. Thus, the usage efficiency of light can be raised, so that the brightness of the image can be raised without raising the brightness of the light source.
FIG. 16 is a sectional view of a liquid crystal panel including a microlens array. A TFT substrate 21, a pixel TFT 23, a pixel electrode 22, an orientation film 31, a liquid crystal 24, a counter electrode 25, a BM 26, and a counter substrate 28 are provided as shown in the drawing.
A microlens array 27 including a plurality of microlenses 30 is provided at a side opposite to the TFT substrate 21 across the counter substrate 28. Although the microlens array 27 is provided as if it is in contact with the counter substrate 28 in FIG. 16, it may be provided apart from the counter substrate 28.
One microlens 30 is provided so as to correspond to one pixel, and the size of the microlens 30 is determined by the pixel pitch.
The light incoming from the side of the counter substrate 28 is condensed by the microlens 30 and enters an opening portion 29 of a pixel.
FIG. 17 is a sectional view of the microlens 30. The light incoming from a spherical surface of the microlens 30 is refracted and passes through a focal point O. A distance between a principal point O′ of the microlens and the focal point O is a focal distance f. In the microlens shown in the drawing, although the principal point is an apex of the sphere of the microlens, the position of the principal point becomes different according to the shape of the microlens.
When the microlens 30 is regarded as a part of a sphere, the center of the sphere is made a center C, and its radius is made a radius of curvature r.
Since a diameter D of the microlens is determined by a pixel pitch of a corresponding pixel, when the area of the pixel portion of the liquid crystal panel is reduced, it is necessary that the diameter D of the microlens is also reduced.
In order to decrease the diameter D of the microlens, there are a method of decreasing the diameter while similar figures are kept without changing the radius of curvature r, and a method of decreasing the radius of curvature r.
The former is not easy in design and manufacture, and it is difficult to increase the number (integration) of microlenses per unit area of the microlens array.
An F value of a lens is a value obtained by dividing a focal distance by a diameter. Lenses having the same radius of curvature have the same focal distance. Thus, if the diameter D is made small while similar figures are kept without changing the radius of curvature r, the F value becomes large, and the amount of light per unit area reaching an image plane becomes small, which is not preferable.
The latter method of decreasing the diameter D by decreasing the radius of curvature r is relatively easy in design and manufacture as compared with the former method. However, if the radius of curvature r is made small, the focal depth becomes shallow, and it becomes difficult to effectively condense light to the opening portion of the pixel. The reason will be described below in detail.
The focal depth is a movement distance of an imaging plane in which required resolution is satisfied when the imaging plane moves in an optical axis direction. The focal depth T is obtained by the following expression 1 from the required resolution S and the F value.T=2×S×F  [Expression 1]
In this case, the required resolution S is in proportion to the size of the opening portion of the pixel, and when the opening portion is large, that is, the pixel pitch is large, the required resolution S becomes large. On the contrary, when the opening portion is small, that is, the pixel pitch is small, the required resolution S becomes small.
The F value is obtained by the following expression 2 from the focal distance f and the diameter D of the microlens.F=f/D  [Expression 2]
The diameter D of the microlens is in proportion to the pixel pitch, and like the required resolution S, when the pixel pitch is large, the diameter D becomes large, and when the pixel pitch is small, the diameter D also becomes small.
The focal distance f is obtained by the following expression 3 from the radius of curvature r of the microlens, and a constant n determined by the refractivity of the microlens and the refractivity of a medium.f=nr  [Expression 3]
When the focal depth T is obtained from the expressions 1 to 3, the following expression 4 is derived.T=(2n×r)·S/D  [Expression 4]
Here, the required resolution S and the diameter D are values determined by the pixel pitch and the size of the opening portion, and have the same first-order parameter. Thus, from the expression 4, it is understood that the focal depth T is determined by the radius of curvature r. When the radius of curvature r of the microlens array is made large, the focal depth T also becomes deep. On the contrary, when the radius of curvature r is made small, the focal depth T also becomes shallow.
The upper surfaces of the TFT substrate and the counter substrate are not completely flat. Thus, in the case where a cell gap is irregular over the whole substrate, even if there is no problem when the diameter D of the microlens is large, there has been a problem that uneven brightness of an image is seen when the diameter D is made small. Then, it is required to make the cell gap more uniform.
Besides, since the microlenses are formed on one surface of a microlens array substrate, the microlens array substrate is not flat, but a warp is produced. In the case where the microlens array is bonded to the counter substrate through an adhesive such as an ultraviolet ray curing resin, because of unevenness in a hardening time of the adhesive, contraction of the ultraviolet ray curing resin at the time of hardening, and hardening of the ultraviolet ray curing resin in the state where an applied pressure at the time of bonding remains, a warp is produced in the counter substrate after the bonding. Further, in the case where thermal expansion coefficients of the microlens array substrate and the counter substrate are different from each other, a warp of the substrate due to a temperature chance is produced. Besides, in the case where a thin substrate is used from the viewpoint of achievement of light weight and low cost, since the substrate lacks rigidity, the warp of the substrate is produced. As a result, there has been a problem that the cell gap becomes irregular and uneven color is produced. Then, it is required to make the cell gap more uniform.
In the case where spherical spacers are provided between the TFT substrate and the counter substrate, it is possible to remove a difference (error) in the cell gap due to places on the substrate to a certain degree. However, in future, since it is necessary to fabricate a liquid crystal panel with a pixel pitch of 40 μm or less, preferably 30 μm or less, when the pixel pitch becomes small, even the spherical spacer of several μm results in deterioration of display quality when it exists at an opening portion of a pixel.
FIG. 18 is a schematic view of a pixel using a spherical spacer. A wiring 42, a pixel TFT 45 including an active layer 43 and a gate electrode 44 as a part of the wiring 42, and a pixel electrode 46 are provided as shown in the drawing. A BM 47 is provided over the wiring 42 and the pixel TFT 45 to cover a region which is not required to transmit visible light.
When a spherical spacer 49 is positioned on the pixel electrode 46 of an opening portion 48, since the orientation of a liquid crystal material is disturbed in the vicinity of the spherical spacer 49, there is a case where a disturbance (disclination) of an image display is observed.
Similarly, even in the case where the spherical spacer 49 is provided on the wiring 42, when the spherical spacer 49 is close to the opening portion, there is a case where the disclination 50 is observed.
Besides, the upper surfaces of the TFT substrate and the counter substrate themselves are not completely flat. Thus, even if the spherical spacers are scattered on the upper surface of the TFT substrate, the cell gap becomes different according to the places on the substrate, and it is impossible to realize a uniform cell gap over the whole substrate. As a result, a deformation is produced in the counter substrate. In a liquid crystal panel in which the difference of the cell gap due to the places on the substrate is produced or the deformation is produced in the counter substrate, there appear such defects that uneven display occurs or interference fringes occur on the upper surface of the counter substrate.
Further, in the conventional spherical spacers, when a liquid crystal material is injected, the spherical spacers themselves flow by the flow of the liquid crystal material, and consequently, uniform spacer scattering density can not be obtained, and the spacers can cause the difference in the cell gap due to the places on the substrate.
A generally manufactured or experimentally manufactured liquid crystal panel secures a cell gap of about 4 to 6 μm irrespective of a pixel pitch. Besides, in a liquid crystal panel using a ferroelectric liquid crystal which has attracted attention recently, a small cell gap is required from its characteristics.
However, it is generally difficult to fabricate a cell having a small and uniform cell gap by using the conventional spherical spacer.
As described above, in the case where the cell gap is controlled by using the conventional spherical spacer, there is a problem that it is difficult to obtain an excellent display due to various factors.